Interaction between the bacterial flora resident in dental plaque and the host is being studied, with emphasis upon identifying and describing immunological reactions which form part of the pathogenetic mechanism. Clinical studies of humans are supplemented with observations on an animal model. The monkey, M. speciosa, which we employ has a plaque and crevice flora like the human, and its oral environment is comparable. Humoral and cell-mediated immunity to plaque bacteria was found, and its relation to plaque-induced experimental disease is being delineated. Studies of man include: analysis of plaque and gingival crevice microbiology; examination of gingival crevice of the pathogenetic and prognostic significance of specific immune reactions with selected antigens.